This invention relates to a mechanical press used to convert shells into ends for self-opening cans and the like and, more particularly, to a belt and belt drive for conveying the shells through such a conversion press.
Soft drinks, beer, juices and the like, are typically packaged in cans formed from aluminum or coated steel. The can body is manufactured by several known methods to include sidewalls, a bottom, a top neck to which an end is attached after the body is filled. The upper end or top, which may include means by which the can is later opened, is of course manufactured separately. These so-called self-opening or "pop top" ends are made from a shell (the principal component of the end) which is subsequently converted to an end by appropriate scoring and attachment of a tab by known integral riveting techniques in another press.
U.S. Pat. No. 4,568,230, assigned to the assignee of this application, discloses a typical conversion press for scoring shells and attaching tabs thereto. This press includes a conveyor belt which extends from one side of the press to the other through in-line conversion tooling. Cooperating with the conveyor are upstacker and downstacker mechanisms located on either side of the slide, to supply shells to and remove shells from the conveyor belt. In some applications upstackers may not be used, rather the ends may be discharged from the end of the conveyor belt. The shells are received in circular apertures in the conveyor belt, which is moved stepwise through the press in synchronism with the opening and closing of the tooling.
As shown in FIGS. 3, 6 and 10 of U.S. Pat. No. 4,568,230, a strip for forming tabs to be attached to the shells is conveyed across the path of the shells in the conveyor belt. The tab strip is conveyed through the press in a generally front to rear direction and tabs are formed in the tab strip as it is conveyed through tab forming stations within the press, while the shells are conveyed simultaneously to successive tooling stations where various forming and scoring operations are performed. The tab strip and shells meet at a tab attachment station where the completed tabs are transferred from the tab strip to the shells to form completed can ends.
In commercial versions of conversion presses of the type shown in said U.S. Patent a stainless steel conveyor belt has been used, and has been provided with a plurality of rectangular sprocket holes extending in a longitudinal direction along either edge of the conveyor belt. In presses for conversion of beer/beverage can ends, three lanes of shell-receiving apertures or pockets are provided, with the pockets in each lane offset lengthwise of the belt from those pockets in the adjacent lane. This spatial relationship is dictated by the geometry of the several lanes of tooling in the press, it being understood that the center-to-center distance along each lane between the pockets is the same, and equals the distance the conveyor belt must advance between successive closures of the tooling, to locate the shells concentrically between the successive tooling stations. In presses for conversion of larger ends, as for food cans, there may be only two lanes, with the pockets alternating lengthwise of the conveyor belt, and the incremental movement of the belt will be correspondingly greater.
A drive drum supporting the conveyor belt at one end thereof has been provided with a plurality of generally rectangular sprocket teeth for engaging in like shaped sprocket holes formed along the edges of the belt, thereby to provide positive engagement between the drive drum and the belt for accurately displacing the shells in their intermittent movement through the press. In a typical such conveyor belt arrangement (for beer/beverage can ends), the belt is 10 inches wide, and sprocket teeth have a 0.5 inch (1.27 cm) on center spacing and the shell receiving apertures or pockets have a 3 inch (7.62 cm) on center spacing in the longitudinal direction such that approximately six sprocket teeth are provided for every two shell receiving apertures on the conveyor belt.
Conversion presses of this type will have design speeds in the order of 400 to 600 strokes/minute, sometimes even higher. The power for the conveyor drive is usually derived via a power take-off mechanism from the main press drive, wherein one revolution of the main drive is translated into a single stroke of the press tooling. This mechanism is commonly called an "intermitter." To avoid interference between conveyor belt motion and the closing-opening action of the tooling, indexing (incremental advancing) of the conveyor belt is generally confined to about 210 degrees of crankshaft rotation, leaving a dwell of 150 degrees in the conveyor drive, divided around bottom dead center of crank rotation. In a typical beer/beverage conversion press with center-to-center belt aperture spacing of 3 inches (76.2 mm.), this of course equals the length of the indexing motion of the conveyor belt. Thus, at a press speed of 600 rev./min. the complete indexing motion must occur in approximately .06 seconds.
A similar conversion press for manufacturing ends for food cans presents the same situation on a somewhat different scale. The shells have a center-to-center spacing in the order of 3.5 inches (as mentioned), thus the belt indexing mechanism is set to increment that distance. A press with this arrangement typically has a design operating speed in the order of 450 rev./min.
It follows that the forces required to accelerate and decelerate the conveyor system are substantial, and there is considerable stress in the belt at the edges of sprocket holes, at a time when the belt is flexing as it wraps around the drive cylinder.
As a result of these forces being transmitted from the sprocket teeth on the drive drum to the edges of the rectangular sprocket holes in the conveyor belt, these conveyor belts have been subject to stress failures, e.g. cracking occurring at the location of the sprocket holes. When such a failure occurs in a conveyor belt, the press must stop, the belt is cut and removed from the press, then a new belt installed and then welded (in the case of the steel belt) into an endless loop, and the belt drive tightened.
Thus, there is a need for a conveyor belt design for use with a conversion press in which the stresses applied at the drive holes on the belt are minimized. There is also a need for a conveyor belt design in which the drive pins or teeth on the drive drum cooperate with a portion of the conveyor belt which has a high resistance to stress failures, such that the useful life of the conveyor belt is lengthened.